1. Field of the Invention
The present invention relates to electrically rewritable semiconductor storage devices, and in particular to, among these, a non-volatile semiconductor storage device and a method of manufacturing the same.
2. Description of the Related Art
Conventionally, LSIs are formed by integration of devices in a two-dimensional plane on the silicon substrate. Although the dimension for each device must be reduced (refinement) to increase memory storage capacity, recent years are facing challenges in such refinement from the viewpoint of cost and technology. Such refinement requires further improvements in photolithography technology. However, in currently available ArF immersion lithography technology, for example, the resolution limit has been reached around the 40 nm design rule and so EUV exposure devices have to be introduced for further refinement. However, the EUV exposure devices are expensive and infeasible in view of the costs. In addition, if such refinement is accomplished, it is assumed that physical improvement limit, such as in breakdown voltage between devices, would be reached unless driving voltage can be scaled, for example. Accordingly, it is likely that difficulties would be encountered in device operation itself.
Therefore, a large number of semiconductor storage devices have been proposed recently where memory cells are arranged in a three-dimensional manner to achieve improved integration of memory devices (see, Patent Document 1: Japanese Patent Laid-Open No. 2003-078044; Patent Document 2: U.S. Pat. No. 5,599,724; and Patent Document 3: U.S. Pat. No. 5,707,885).
One of the conventional semiconductor storage devices where memory cells are arranged in a three-dimensional manner uses transistors with a SGT (cylinder-type) structure (see, Patent Documents 1-3). Those semiconductor storage devices using transistors with a SGT (cylinder-type) structure are provided with multiple layers of polysilicon corresponding to gate electrodes and pillar-like columnar semiconductors. Each of the columnar semiconductors serves as a channel (body) part of each of the transistors. A plurality of charge accumulation layers are provided around the columnar semiconductors via tunnel insulation layers for accumulating charges. Further, block insulation layers are formed around the charge accumulation layers. Such configurations including polysilicon, columnar semiconductors, tunnel insulation layers, charge accumulation layers, and block insulation layers are referred to as “memory strings”.
In the aforementioned conventional technology, columnar semiconductors are first formed, around which tunnel insulation layers, charge accumulation layers, block insulation layers, and gate electrodes are sequentially formed. Such way of formation, however, requires a large number of manufacturing steps and thus becomes cumbersome.